The present invention relates to the use of certain blocked polyisocyanates as crosslinking agents for organic fixing agents used in textile printing.
In the textile printing industry the use of externally crosslinkable film-forming fixing agents, for example, those based on polyacrylate- or butadiene-acrylonitrile copolymers, is widespread. The mixtures of fixing agent and pigment usually also contain reactive compounds, which crosslink the fixing agent at a higher temperature after the printing operation and are thereby able to fix the print. (As used herein, the term "pigments" also encompasses dyes.) It is only this fixing process that results in the desired color fastness in water, which is important, for example, when the printed textiles are washed.
In practice, melamine/formaldehyde condensation products, which participate in crosslinking reactions only at temperatures above about 120.degree. C., are still always used as the crosslinking agents for the fixing agents employed in textile printing pastes. According to W. Berlenbach in Ullmanns Encyklop adie der technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], Fourth Edition, Volume 22 (Verlag Chemie, Weinheim, 1982), page 629, crosslinking via N-methylol groups of the fixing agent is catalyzed by acid; the best fixing results are obtained using dry hot air over a period of 5 to 10 minutes at 140.degree. C. or 30 to 60 seconds at 175.degree. C., for example.
Textile printing pastes proposed in German Offenlegungsschrift 3,529,530 contain externally crosslinkable fixing agents and deactivated (that is, partially blocked) polyisocyanates as crosslinking agents dispersed in water or in organic solvents. According to the claims of this patent, these fixing agents comprise solid polyisocyanates having melting points above 470.degree. C. For example, dimeric toluene diisocyanate having a melting point of 170.degree. C. is used.
German Offenlegungsschrift 3,109,978 describes aqueous textile printing pastes that contain film-forming, externally crosslinkable polymers as fixing agents and NCO prepolymers as crosslinking agents. Compared with low molecular weight polyisocyanates, these prepolymers apparently impart a prolonged pot life to the printing pastes because of their higher molecular weights and their reduced content of reactive groups. However, this is insufficient under practical conditions; in particular, the finished printing pastes are not stable on storage.
It has also been proposed that polyisocyanate mixtures comprising hydrophilic polyisocyanates or hydrophilic isocyanate derivatives containing carbodiimide groups be used as crosslinking agents for fixing agents employed in textile printing. See German Offenlegungsschrift 4,217,716. However, the storage stability of printing pastes based on aqueous dispersions is also unsatisfactory, because a continuous loss of isocyanate groups takes place through reaction of free isocyanate groups with water, which ultimately leads to products which are inactive with respect to crosslinking.
The prior art textile printing pastes described above are subject to various disadvantages. For example, formaldehyde is given off during the crosslinking reaction or even later when the printed textiles are used, which is particularly undesirable, or the number of reactive groups available for crosslinking is difficult to control, or the crosslinking agent is difficult to disperse.
The object of the present invention, particularly for environmental and health reasons, was to prepare formaldehyde-free crosslinking agents for printing pastes that are; stable on storage and can be processed to form textile prints of high color fastness. In this respect, the term "formaldehyde-free" refers to the absence of disadvantage related to the emission of formaldehyde over the course of time, which is an undesirable property of products containing methylol groups. In the sense of the invention, the term "stable on storage" not only means that the dispersions should not segregate but, most importantly, also means that the crosslinking agents do not lose their reactivity during storage and thus after storage give prints having an undiminished, pronounced color fastness.
It has now surprisingly been found that the object of the invention can be achieved by using certain blocked polyisocyanates. Thus, printing pastes having storage stabilities exceeding four weeks and prints having high levels of dry- and wet-rubbed color fastness (more than 100 brush washings) become accessible.